1. Field of the Invention
The present invention relates to an antenna apparatus for use in a wireless communication apparatus that operates in mobile communication using a mobile phone, etc. so as to increase communication capacity, achieve high-speed communication, as well as maintain high communication quality. More particularly, the present invention relates to a MIMO antenna apparatus, and a wireless communication apparatus provided with the MIMO antenna apparatus.
2. Description of the Related Art
An antenna apparatus disclosed in Patent Document 1 and an adaptive array wireless apparatus disclosed in Patent Document 2 have been known as antenna apparatuses each using a plurality of antenna elements and selectively changing them (diversity antenna apparatuses).
The antenna apparatus disclosed in Patent Document 1 is provided with an antenna element for transmitting and receiving radio waves. The antenna element has a first feed position and a first ground position at a certain part, and a second feed position and a second ground position at another part. The antenna element is further provided with switching means for switching, during feeding to the antenna element, between the feeding by grounding at the first ground position and feeding through the first feed position, and the feeding by grounding at the second ground position and feeding through the second feed position. Particularly, the antenna apparatus configured as an antenna part of a mobile phone is provided with an antenna element serving as a rod element with the configuration and function of an inverted-F antenna. The rod element has a first feed position and a first ground position at one end thereof, and has a second feed position and a second ground position at the other end thereof. A switch unit (switching means) switches between the two feed positions, for feeding the rod element. The antenna apparatus implements a function as two antennas with different directional characteristics, by switching between the two feed positions through the switch unit. According to the antenna apparatus disclosed in Patent Document 1, it is possible to reduce the antenna size, and to provide a diversity antenna capable of improving a diversity effect by switching between two feed positions on an antenna element of a rod element for changing radiation directivity.
Furthermore, the adaptive array wireless apparatus disclosed in Patent Document 2 has an array antenna including a group of three or more antennas. The adaptive array wireless apparatus includes: antenna selecting means for selecting a combination of two antennas from among the group of antennas according to an antenna selection signal; two signal receiving means for performing certain radio signal processing on signals received by the selected combination of antennas; adaptive array signal processing means for performing an adaptive array receiving process on the signals subjected to the radio signal processing by the signal receiving means, and extracting a desired signal; antenna selection controlling means for controlling the antenna selection signal to change a combination of two antennas to be selected by the antenna selecting means; correlation value estimating means for estimating and recording an antenna correlation value of the signals subjected to the radio signal processing by the signal receiving means, for each combination of two antennas selected by the antenna selecting means; and antenna combination determining means for determining a combination of two antennas with the minimum antenna correlation value, and causing the antenna selection controlling means to select the determined combination of two antennas. Thus, an improvement in adaptive array receiving performance can be achieved without providing additional signal receiving means. That is, according to the configuration disclosed in Patent Document 2, an adaptive antenna is implemented, including two signal receiving means and controlling the switching among antennas. Accordingly, both low power consumption and a small size can be achieved.
(1) Patent Document 1: Japanese Patent Laid-Open Publication No. 2004-40554, and
(2) Patent Document 2: Japanese Patent Laid-Open Publication No. 2004-289407.
On the other hand, there are antenna apparatuses that adopt MIMO (Multi-Input Multi-Output) technique for transmitting and receiving radio signals of a plurality of channels simultaneously by using a plurality of antennas.
When using the antenna apparatuses disclosed in Patent Documents 1 and 2 as MIMO antenna apparatuses, the following problems arise.
The diversity antenna disclosed in Patent Document 1 is configured to detect only the magnitudes of the received signals and select the feed position with a higher received power, in order to obtain as high received power as possible. However, in MIMO antenna apparatuses, even when received radio signals have high received powers, if there are large differences among the received powers, a MIMO demodulation result degrades. Furthermore, in MIMO antenna apparatuses, when the correlation coefficients between antennas are high as much as one, MIMO demodulation cannot be achieved. That is, in MIMO antenna apparatuses, it is extremely difficult to improve MIMO demodulation performance by performing antenna selection based on received powers only.
On the other hand, the adaptive array wireless apparatus disclosed in the Patent Document 2, that selects two antennas from among the plurality of antennas, is configured to select antennas to be connected to a demodulator circuit, according to only the correlation coefficients between the antennas, in order to improve an interference wave suppression effect. However, since MIMO antenna apparatuses use a technique of spatially multiplexing a plurality of signals for implementing high-speed wireless communication, they require not only low correlation coefficients between antennas, but also high received powers to achieve low error rate. Namely, it is extremely difficult to improve MIMO demodulation performance by performing antenna selection based on correlation coefficients only.